1. Field of the Invention
The present invention generally relates to a method of fabricating a light emitting diode (LED); more specifically, to a method of fabricating a light emitting diode by cyclic ramping of a process temperature.
2. Description of Related Art
In recent years, due to improvements in the luminescence efficiency of the light emitting diodes (LEDs), LEDs have begun to gradually replace the fluorescent lamps and the incandescent bulbs in some applications, such as light source of the scanner where rapid response is paramount, the backlight source for the liquid crystal display (LCD), and the light source for general illumination devices. Since the III-V nitrides are semiconductor materials with wide band gap, the light emitted therefrom encompasses almost the entire visible and ultraviolet spectrum. Consequently, III-V materials such as gallium nitride (GaN) have been widely used in LEDs.
FIG. 1 is a cross-sectional view illustrating a conventional LED. FIG. 2 is a process temperature curve diagram for the LED depicted in FIG. 1. FIG. 3 is an X-ray diffraction spectrum diagram for a light emitting layer of the LED depicted in FIG. 1. Referring to FIG. 1, the conventional LED 100 includes an n-type GaN layer 110, a p-type GaN layer 120, a light emitting layer 130 disposed between the n-type GaN layer 110 and the p-type GaN layer 120, and a substrate 140. The n-type GaN layer 110, the light emitting layer 130, and the p-type GaN layer 120 are stacked on a surface 142 of the substrate 140.
The light emitting layer 130 is fabricated by alternately forming a plurality of GaN layers 132 and a plurality of indium gallium nitride (InGaN) layers 134, where the GaN layers 132 are barrier layers and the InGaN layers 134 are quantum well layers. Furthermore, only a portion of the n-type GaN layer 110 is covered by the p-type GaN layer 120 and the light emitting layer 130. Two electrodes E are respectively disposed on and electrically connected to the p-type GaN layer 120 and the n-type GaN layer 110.
Conventionally, metal organic chemical vapor deposition (MOCVD) is used to form the n-type GaN layer 110, the p-type GaN layer 120, and the light emitting layer 130 of the LED 100. More specifically, referring to FIG. 1 and FIG. 2, first a process temperature is maintained at a first growth temperature TH, and the n-type GaN layer 110 is formed on the surface 142 of the substrate 140 at the first growth temperature TH. Next, the process temperature is ramped down to a second growth temperature TL, where TH>TL, and the light emitting layer 130 is formed on the n-type GaN layer 110 at the second growth temperature TL. Finally, the process temperature is ramped up to a third growth temperature, and the p-type GaN layer 120 is formed at the third growth temperature.
It should be noted that, if the process temperature is substantially high during a growing process of the InGaN layers 134 within the light emitting layer 130, the substantially high growth temperature may cause precipitation of indium. Therefore, to avoid precipitation of indium, the light emitting layer 130 is formed at a second growth temperature lower than the first growth temperature TH. However, as shown in FIG. 3, the X-ray diffraction spectrum peak of the GaN layers 132 formed at the second growth temperature TL has a wide full width at half maximum (FWHM). In other words, the GaN layers 132 formed at the second growth temperature TL has poor epitaxy quality. Moreover, optical characteristics of the LED 100 are affected by such epitaxy quality.